1. Field of the Invention
The present invention relates to method and equipment for producing a protective-coated steel pipe having a thermoplastic synthetic resin coating formed on the outer peripheral surface thereof for use, for example, as a foundation pile in civil engineering works.
2. Description of the Prior Art
Some of the structures such as bridges and piers built on the sea are supported on bearing piles which are driven into the bottom of the sea. In this case, steel pipe posts are disposed between such a structure and the tops of the bearing piles. The portions of the steel pipe posts which are in contact with the surface of the sea, as well as the portions thereof positioned thereabove, are exposed alternately with sea water and air due to ebb and flow of the tide and through splashes of waves, so those portions are most likely to be corroded. Therefore, anticorrosive measures for those portions are particularly important. The following are examples of conventional corrosionproof steel pipes used for this purpose:
(1) a corrosionproof steel pipe with a urethane- or epoxy-based coating material applied as thick as 3 mm or so; PA0 (2) a protective-coated steel pipe comprising a steel pipe and a coating (e.g. a melt-extruded coating of polyethylene) formed on the outer peripheral surface of the steel pipe by extruding molten polyolefin resin continuously onto the same surface; and PA0 (3) a spiral tape wound pipe of which tape winding is prepared in a construction work site such as that disclosed in Japanese Patent Laid-Open No. 40925/86.
Such conventional corrosionproof steel pipes involve the following problems.
In the pipe (1), the cost of the coating material is high.
In the case of the pipe (2) according to the polyethylene melt-extrusion coating method, the manufacturing equipment is very expensive and the cost of transport of the steel pipe after anticorrosion processing is high because vulnearable coating must be enoughly protected during delivery. Further, the pipe may be flawed during handling in transit, thus requiring considerable labor for its repair.
According to the above technique (3), a heat-shrinkable polyolefin tape having an adhesive layer formed of a thermoadherent polyolefin which has an adherent functional group is wound spirally around the peripheral surface of the steel pipe pile while being partially overlapped at side edges portions thereof. In such a steel pipe pile, a gap is formed between the overlapped part of the tape and the pipe, which gap causes corrosion. And it is difficult to form the tape winding start and end in good shape.
Further, in seasons or districts wherein the climate temperature is low, the heat-shrinkable polyolefin tape hardens and loses flexibility out of doors. Consequently, when pushed against the surface of the steel pipe, the tape does not fit closely on the surface, and even when the steel pipe is preheated to a predetermined temperature, the shape of the tape just after pressure bonding thereof to the pipe is unstable and there remains a gap between the tape and the steel pipe. This affects the pipe even after pressure bonding of the tape to the pipe and prevents the tape from forming a uniform coating.